The present invention relates to a carbon heating element having an arbitrary specific resistance and an arbitrary shape which are necessary, arbitrary as a heating element, and a method of producing the same.
Worked materials of metal wire such as tungsten wire and Nichrome wire, machined materials of carbon such as isotropic carbon material and glassy carbon, and metal compounds such as silicon carbide have heretofore been used, principally, as heating element resistances. Of these substances, the worked material of metal wire has been mainly used as a heating element for heaters in a small sized commercial apparatus, and the carbon and metal compounds have been used for industrial furnaces, etc.
Of conventional materials for heating elements, carbon differs from metal wire, etc. in that it is excellent in properties such as a heating rate, heating efficiency and the efficiency of generating far infrared rays. However, because conventional carbon heating elements are produced from large plate-like or block-like bodies by machining, the production process is complicated and costly, and production of thin rods and sheets is difficult. Moreover, the heating elements have a problem in that there are no measures other than to vary the shape of the elements to control the calorific values of the elements because the heating elements are prepared by cutting blocks, etc., having specific resistances in a certain specified ranges.
The present invention has been achieved in view of such problems. An object of the present invention is to provide a carbon heating element the heating of which can be controlled by applying a predetermined current and a predetermined potential in broad ranges because the heating element can be made not only in a sheet-like form but also in a thin rod-like form and a thin cylindrical form that cannot be obtained when the heating element is made of a conventional carbon material and because the heating element can be made to have an arbitrary specific resistance, an excellent heating rate, an excellent heating efficiency and excellent efficiency in generating far infrared rays, and a method of producing the same.
In view of the situation described above, the present inventors have intensively carried out research on the development of a heating element having an arbitrary specific resistance and an arbitrary shape which are necessary to a heating element. Consequently, the present inventors have confirmed the fact that a carbon heating element obtained by mixing, for the purpose of making the heating element have a desired resistance after firing and carbonizing, one or at least two metal or metalloid compounds such as metal carbides, borides, silicides, metal nitrides, metal oxides, metalloid nitrides, metalloid oxides and metalloid carbides with a composition having shapability and showing a substantially nonzero yield of a carbon residue after firing, and firing the resultant mixture can effectively solve the above problems. That is, the carbon heating element has a specific resistance and a shape which are arbitrary, and the heating of the heating element can be controlled by a predetermined current and a predetermined potential; moreover, the heating element is excellent in heating rate, heating efficiency and the efficiency of generating far infrared rays.
The present invention provides a method of producing a carbon heating element, which comprises the steps of mixing a composition having shapability and showing a substantially nonzero yield of a carbon residue after firing with one or at least two metal or metalloid compounds, and firing the mixture.
The present invention also provides a carbon heating element produced by the method mentioned above.
Examples of the metal or metalloid compounds mentioned above include metal carbides, borides, silicides, metal nitrides, metal oxides, metalloid nitrides, metalloid oxides and metalloid carbides. The types and amounts of metal compounds and metalloid compounds to be used are suitably selected in accordance with the resistance and shape of a desired heating element. Although a single compound or a mixture of at least two of the compounds can be used, use of boron carbide, silicon carbide, boron nitride and aluminum oxide is particularly preferred in view of easy control of the resistance. In order to maintain the excellent properties carbon has, the amount to be used is preferably up to 70 parts by weight.
Organic substances showing a yield of carbonization of at least 5% when fired under an inert gas atmosphere are used as a composition mentioned above. Concrete examples of the organic substances include thermoplastic resins such as polyvinyl chloride, polyacrylonitrile, polyvinyl alcohol, vinyl chloride-vinyl acetate copolymer and polyamide, thermosetting resins such as phenolic resin, furan resin, epoxy resin, unsaturated polyester resin and polyimide, natural polymers having condensed polycyclic aromatic groups in a basic structure thereof, such as lignin, cellulose, tragacanth gum, gum arabi and saccharide, formalin condensation products of naphthalenesulfonic acid which are not included in the substances mentioned above, and synthetic polymers having condensed polycyclic aromatic groups in a basic structure thereof, such as copna resin. The type and amount of a composition to be used are suitably selected in accordance with the shape of a desired heating element. The organic substances can be used singly or in a mixture of at least two of them. Use of a polyvinyl chloride and furan resin is particularly preferred. In order to maintain the excellent properties carbon has, the amount of the resins to be used is preferably at least 30 parts by weight.
The composition preferably contains carbon powder. Examples of the carbon powder-include carbon black, graphite and coke powder. The types and amounts of carbon powders to be used are suitably selected in accordance with the resistance and shape of a desired heating element. The carbon powders can be used singly or in a mixture of at least two of them. However, use of graphite is particularly preferred because of the easy control of the shape.
In the present invention, the carbon material produced by firing the organic substances as mentioned above and the carbon powder act as good conductors, and the metal or metalloid compounds act as conductivity-inhibiting materials. The current jumps over, namely, hops over the metal or metalloid compounds which are conductivity-inhibiting material, and flows through the carbon material, or the carbon material and carbon powder as a medium. The carbon heating element of the present invention having a desired specific resistance can therefore be obtained by varying the types and proportion of these two or three components, mixing and dispersing these components, and firing the mixture.
Furthermore, because the carbon heating element of the present invention is excellent in properties as a heating element such as a heating rate, heating efficiency and the efficiency of generating far infrared rays, and because it can be made to have a resistance and a shape which have been designed in advance, it is possible to control the calorific value easily by applying a current and a potential which have been predetermined.
However, when the calorific value is to be controlled, the heating element may sometimes have a considerably high temperature. Oxidation of the heating element must therefore be prevented by using it in a container having an atmosphere of an inert gas such as an Ar gas. Moreover, it is desirable to use a transparent container such as a quartz container not impairing the efficiency of generating far infrared rays and capable of withstanding the high temperature.
The method of producing the carbon heating element according to the present invention will be explained below. First, a composition is mixed well with metal or metalloid compounds using a kneader. The mixture thus obtained is shaped into a designed form by a conventional procedure such as a vacuum forming machine, an injection molding machine or an extruder. The shaped material is subsequently treated to give a precursor of carbon. The precursor thus obtained is heated to about 1,000xc2x0 C., preferably about 2,000xc2x0 C., under an atmosphere of an inert gas such as argon or in vacuum to be carbonized, thereby producing a carbon heating element. It is suitable that the precursor be slowly fired particularly in the temperature range of up to 500xc2x0 C. at a rate of 3 to 100xc2x0 C./h, preferably 5 to 50xc2x0 C./h. When the heating rate is large, the fired material is deformed or defects such as fine cracks are formed therein. Accordingly, a heating rate of at least 100xc2x0 C./h should be avoided in the temperature range of up to 500xc2x0 C.
The carbon heating element of the present invention is excellent, as a heating element, in properties such as a heating rate, heating efficiency and the efficiency of generating far infrared rays, and can be made to have a resistance and a shape which have been designed in advance. It is therefore possible to control the calorific value easily by applying a predetermined current and a predetermined potential.